The risk to the fetus from exposure to low levels of ionizing radiation is of continued concern. Permanent damage to the central nervous system has been shown to be produced by doses as low as 0.25 Gy. However, more information is needed to define the gestational period of maximum sensitivity and the functional outcome of neuronal damage. The rat model will be used to evaluate the effects of gamma radiation doses between 0 and 1 Gy delivered to the pregnant rat between gestational days 14 - 17. Specific alterations in the rat brain will be investigated at three time points: 1) Gestational days 14-18: Evaluation of neurons in the developing fetal cortex at 0 - 72 hr post-irradiation to document the immediate cellular changes that occur, as well as the repair of that damage. Standard morphometric analyses (including autoradiography and immunocytochemistry) will be used. 2) Post-natal, 2 - 28 days: Early (pre-weaning) functional deficits in central nervous system activity identified by behavioral tests will be correlated with body weight and thickness of the sensorimotor cortical layers at postnatal day 28. 3) Post-natal, 1 - 6 months: Progressive behavioral changes will be correlated with morphological maturation of the brain. Three specific aims will be addressed. First, a detailed morphological examination will be conducted of the acute (1-72 hr post-irradiation) response of the fetal cerebral cortex. Second, statistical relationships will be explored between postnatal functional and morphological deficits, especially as the brain progressively develops and matures. Third, dose- response curves (both morphological and functional) will be developed to provide answers to the issue of threshold doses for single and fractionated doses of radiation. The information gained will contribute to our understanding of the effect of ionizing radiation on cells and tissues that might potentially result in both subtle and drastic alterations in the developing mammalian central nervous system.